Many (most ?) modern cars use a fusible link in this position. I would recommend a self-resetting circuit breaker to protect the headlamps.

Note that, disconnecting the alternator under load can blow the diodes in the alternator - still, that is better than a fire! I would think that a 60 amp fuse would be a fairly good starting point. It might be interesting to turn every accessory in the car on (engine off) and measure the actual current flow. You might find that 80 amps would give you a bit more safety margin while still protecting against fire.

I'm not sure what the gauge is for the brown wire but I suspect it is not larger than 8 AWG. If so then the maximum current capacity for a wire of that gauge over a15 foot run is 30 amps. On the chart I looked at it didn't go above 70 amps and that required a #4AWG wire. An 80 amp or even a 60 amp would be too high to protect the wiring. For a great source for determining proper wire gauge and fuse capacity check out: www.engineeringtoolbox.com

In most tables, the concern is voltage drop. So, an 8 gauge wire can only carry 30 amps over 15 feet without having an unacceptable voltage drop. However, see that a 4 foot run of 8 gauge wire can carry 85 amps (with an acceptable voltage drop). It probably could handle more than that without getting too warm.

The purpose of a fuse in this location is to handle shorts that other fuses won't protect. For instance, if something on the brown circuit before it connects to the fuse box were to short, then this fuse would protect it. Equally so for other unfused circuits (like white). I agree that a 60 amp fuse would be preferable, but you need to handle surges as things are switched on. E.g., what happens if you turn on the wipers and the heater fan and the headlamps at the same time. You don't want to blow a fuse on a short term transient load.

I'm thinking of adding the same. Where did you wire the gauge in (i.e., between what two components? battery/alternator???).

Ray, I removed the large brown wire from the starter solenoid (this is the feed from the battery and supplies all of the cars electrical requirements except for the starter motor). The ammeter is wired in between the large brown wire (which goes to all the cars electrical circuits) and the terminal it was removed from (the one that goes directly to the battery).

The circuit breaker (auto reset 50a) is wired close to the battery feed from the solenoid.

I didn't modify any of the cars wiring, I just used a large spade terminal on the feed to the circuit breaker and fabricated a male spade terminal to plug into the existing terminal on the big brown wire. This way it can be put back to original in a matter of seconds.

Mine is a 65 model with a generator, so it's wiring could be somewhat different to a car with an alternator. The ammeter needs to measure everything coming into and out of the battery.

Keith I use parts from a company called Blue Sea Products.
They have maxi fuse holders and surface mount resettable breakers.
On street rods most people prefer the plastic fuse holders.
But on the boats I always use the 285 series breakers.
In my opinion the worst case scenario is using the car to jumpstart another car.
The alternator senses low voltage and full fields, and in the case of Saturn alternators shoots 90 amps into the wiring. Which usually burns the lug on the alternator.
I have never hooked my MG to a SnapOn AVR so have no idea of the amp loads.

Isn't the presence of a battery cutoff switch in most cases not protection enough?
Presuming one adds some other fuses in normal operation to the original fixed 4 fuses and at least 2 or 3 floating ones. I.c. for fans, headlights, pump and OD circuit .

One can add of course racing type extra's as an isolation switch disconnecting under heavy impact, so even when you lose consciousness the main circuit is still broken automatically

In reply to # 3680546 by Donthuis
Isn't the presence of a battery cutoff switch in most cases not protection enough?

IMO, a cutoff switch is only good for storing the car or a convenient way to remove power before doing repairs. There is no way to react quickly enough to prevent wiring damage if an unprotected circuit goes to ground.

But it does let you turn the power off quickly if required.
Whilst these cars don’t have the same level of circuit protection as a new car you need to remember that they’ve lasted 40+!year with what they’ve got. How many new cars will be round in 40 years time. Designing a wiring harness, building it and then testing it to ensure it’s safe and reliable isn’t a trivial task. Many of the mods I’ve seen can introduce more problems than they solve. Some mods are worth doing and easy to implement. Others can increase the risk of failure or fire.

Quote:
In my opinion the worst case scenario is using the car to jumpstart another car.
The alternator senses low voltage and full fields, and in the case of Saturn alternators shoots 90 amps into the wiring. Which usually burns the lug on the alternator.
I have never hooked my MG to a SnapOn AVR so have no idea of the amp loads.

Everybody loves to boost with the boosting car's engine running. This does give extra power from the alternator. BUT it is really hard on the alternator. When the boosted car's starter cuts in, it is practically a dead short. This stresses the boosting car's diodes severely. Often, the alternator will fail several weeks later. By that time, the owner has forgotten that they did the boost, and wonders what caused the alternator to die.

I am following this thread because it relates to an upcoming wiring maintenance project on my 65 MGB. This has the OEM harness, generator and is positive earth. I am considering in-line fusing the red wires off the headlamp switch and the fuel pump branch circuit. I haven't decided whether to fuse the headlamp or ignition circuits. Under a PO the red wire from the light switch to the rheostat shorted to ground when the rheostat rotated. And the bare end of the unused white overdrive circuit feed shorted under the dash, melting it all way back to the hot side of the fuse box. Luckily the igntion circuit wasn't damaged. Replacement of the short red wire restored the interior lighting. Unfortunately no overdrive so that circuit isn't missed.

If we are trying to protect the branch circuits of an OEM wiring harness I think a 50 amp fuse is too much. Most wiring in the harness is #14 and #18. On my car the only bigger wiring is #12 generator output and control box to battery. The above link advises that #14 wire maximum circuit protection is 35 amps (American rating). For #18 wire maximum circuit protection is 20 amp (American rating). Thus the OEM 35/17.5 British rated fuses are suitable for the few protected circuits. Of course if you tap off the battery lug to feed head lamp relays then you can use bigger wires and fuses. Likewise you could fuse the charge wiring with a bigger fuse.

For reference to something more modern I took a look at the very complex wiring diagrams for a 99 Toyota Camry. The battery feed has a fusible link downstream of the starter tap. The FL feeds into something called the fusible link box. In this box is what appears to be a 100A fuse for the alternator. The headlamp circuit appears to be protected by a 40 amp main fuse with 15 amp fuses on the right and left headlamp feeds. These circuit diagrams are really hard to read so I could be wrong.

Let me tell a story here. True story if that matters.
1947 Ford full custom. Chevrolet motor.
We are working on the car the night before a show. The headlights have an intermittent open. I go under the dash and cut the cable ties to the anchors and inspect the harness under the dash. The wires drop down several inches. I started checking the underhood harness.
My friend who owns the car asks me if he can button up the interior.
He did not secure the harness. He pushes the stereo into the dash. Runs into some resistance and slaps the radio into place. A sharp edge on the back of the radio cuts the power wire running from the battery to the fuse box.
This blows the maxifuse I had installed the week before. I think I just picked a 90amp maxifuse because it was handy.
The size of the fuse didn’t matter because it was a 625 CCA battery going direct to ground.
It happened so fast that there was no reaching the battery switch.